Recently, devices made of compound semiconductor material for high-speed, microwave, and power circuit applications have attracted a great deal of attention and played an important role in electronic integrated circuit field. As far as Field Effect Transistor (FET) is concerned, in order to make the devices operate at high frequency, reducing the FET gate's length, raising the carrier mobility, and decreasing the transport time are necessary. All kinds of high-performance devices, such as Metal Semiconductor Field Effect Transistor (MESFET), High Electron Mobility Transistor (HEMT), Doped Channel Field Effect Transistor (DCFET), and δ-doped layer Field Effect Transistor (δ-doped FET), have been successfully developed and widely applied to digital and microwave circuits, which have brought a lot of benefit to the communication industries. For the example of HEMT, because of the low impurity scattering and the high mobility of the Two-Dimensional Electron Gas (2DEG), the mobility of HEMT is improved to make the transconductance so high that it is suitable for high-frequency circuit application. However, there is still an issue about parallel conduction under the positive gate bias in HEMT. As for DCFET, it exploits non-doped large-energy-gap material to make up the Schottky contact of the gate, so that it avoids the parallel conduction problem and has high linearity of output transconductance. Further, DCFET has advantages of high current density and large breakdown voltage of the gate for high-frequency power circuit application. In the structure of δ-doped FET, a δ-doping sheet makes the V-type potential well and quasi-two dimensional electron gas. Further, VGS has less influence on the depletion thickness such that the linearity of transconductance is improved. There are more advantages in δ-doped FET, such as high output current density, large breakdown voltage, controllable threshold voltage, high linearity of transconductance, and so on. However, as far as MESFET, DCFET, and δ-doped FET are concerned, the carriers in the channel are lack of capability of modulation, which causes a large saturation voltage of the drain-source junction and a small operating range of the devices, and thus they are still unsuitable for circuit application.
Remarkably, in the high-tech microwave area of wireless broadband communication, pHEMT has been extensively applied to high-frequency device technologies for advanced low-power PCS cellular phone and fixed-network. Including all kinds of microwave circuit applications from low-frequency 1.8-2.2 GHz (PCS), 2.2-2.4 GHz (3G wireless cell phone), mid-frequency 28-31 GHz (LMDS, VSAT, broadband satellite), to high-frequency 76-77 GHz (auto radar cruise control), all broadband communication devices will use pHEMT in their major components.
Formerly, in the AlxGa1−xAs/GaAs pHEMT material system, the mole ratio x of Al has to be greater than 0.2 so as to produce larger 2DEG. However, due to the technology of the high etching ratio of InGaP and GaAs recently, the reliability of the devices made of the aforementioned pHEMT is getting better. Further, InGaP/GaAs has a feature of larger ΔEv/ΔEg (about 0.6) to constrain the gate's leakage current resulting from the impact ionization. As a result, the InGaP/GaAs material system has been used popularly. As for InGaP/InGaAs pHEMT, its ΔEc is greater than that of InGaP/GaAs, so that InGaP/InGaAs can achieve excellent transistor's characteristics of larger 2DEG and better confinement effect of electron. There are generally two types of pHEMT's. (1) pHEMT of n-InGaP (or n-AlGaAs) gate, and (2) δ-doped pHEMT. In the case of the pHEMT of n-InGaP (or n-AlGaAs) gate, when the gate-source bias is positively high, the n-InGaP (or n-AlGaAs) gate results in the issue of the large energy-gap parallel conduction and the poor linearity of output transconductance, such that it is unsuitable for power circuit application. As to the δ-doped pHEMT, it inserts δ-doped carrier supplying layer into non-doped i-InGaP (or i-AlGaAs) layer to replace n-InGaP (or n-AlGaAs) gate. Because the gate metal is plated onto the large energy-gap i-InGaP, the breakdown voltage of the gate is raised.